Borehole current correction methods and apparatus

ABSTRACT

Methods and apparatus are disclosed for minimizing or eliminating an undesired axial electric current induced along a subsurface borehole in the process of subsurface measurements with transmitter and/or receiver antennas which are substantially time varying magnetic dipoles with their dipole moments aligned at an angle to the axis of the borehole. Some antennas are disposed within the borehole on instruments having a non-conductive support member. One instrument includes a conductive all-metal body with an antenna adapted for induction frequencies. Antenna shields adapted for controlled current flow are also provided with an all-metal instrument. Methods include providing an alternate path for the current along the instrument body. Another method includes emitting a controlled current to counter the undesired current. Another method corrects for the effect of the current using a superposition technique. An embodiment of the instrument includes an antenna disposed between a pair of electrically coupled electrodes. The antenna is disposed on the instrument such that it comprises a tilted or transverse magnetic dipole. Another embodiment of the instrument includes a non-conductive housing with a conductive segment disposed thereon. An antenna is disposed on the instrument about the conductive segment. Another embodiment includes an antenna disposed between two pairs of electrodes with means to measure a voltage at the electrodes when electromagnetic energy is transmitted within the borehole. Yet another instrument includes an antenna disposed between a first pair of electrodes and means to measure a voltage at the elecrodes when electromagnetic energy is transmitted within the borehole. This embodiment also includes means to energize a second electrode pair in response to the voltage measured at the first electrode pair.

1. BACKGROUND OF THE INVENTION

[0001] 1.1 Field of the Invention

[0002] The invention relates to techniques for reducing and/orcorrecting for borehole effects encountered in subsurface measurements.More particularly, the invention concerns methods, and devices for theirimplementation, in which well logging instruments using sources orsensors having a transverse or tilted magnetic dipole are adapted toreduce or correct for undesired electromagnetic effects associated withthe deployment of the instruments in a borehole.

[0003] 1.2 Description of Related Art

[0004] Various well logging techniques are known in the field ofhydrocarbon exploration and production. These techniques typicallyemploy logging instruments or “sondes” equipped with sources adapted toemit energy through a borehole traversing the subsurface formation. Theemitted energy interacts with the surrounding formation to producesignals that are detected and measured by one or more sensors on theinstrument. By processing the detected signal data, a profile of theformation properties is obtained.

[0005] Electromagnetic (EM) logging techniques known in the art include“wireline” logging and logging-while-drilling (LWD). Wireline loggingentails lowering the instrument into the borehole at the end of anelectrical cable to obtain the subsurface measurements as the instrumentis moved along the borehole. LWD entails attaching the instrumentdisposed in a drill collar to a drilling assembly while a borehole isbeing drilled through earth formations.

[0006] Conventional wireline and LWD instruments are implemented withantennas that are operable as sources and/or sensors. In wirelineapplications, the antennas are typically enclosed by a housingconstructed of a tough plastic material composed of a laminatedfiberglass material impregnated with epoxy resin. In LWD applications,the antennas are generally mounted on a metallic support to withstandthe hostile environment encountered during drilling. Conventionallogging instruments are also being constructed of thermoplasticmaterials. The thermoplastic composite construction of these instrumentsprovides a non-conductive structure for mounting the antennas. U.S. Pat.No. 6,084,052 (assigned to the present assignee) describesimplementations of composite-based logging instruments for use inwireline and LWD applications.

[0007] In both wireline and LWD applications, the antennas are mountedon the support member and axially spaced from each other in thedirection of the borehole. These antennas are generally coils of thecylindrical solenoid type and are comprised of one or more turns ofinsulated conductor wire that is wound around the support. U.S. Pat.Nos. 4,873,488 and 5,235,285 (both assigned to the present assignee),for example, describe instruments equipped with antennas disposed alonga central metallic support. In operation, the transmitter antenna isenergized by an alternating current to emit EM energy through theborehole fluid (also referred to herein as mud) and into the formation.The signals detected at the receiver antenna are usually expressed as acomplex number (phasor voltage) and reflect interaction with the mud andthe formation.

[0008] One EM logging technique investigates subsurface formations byobtaining electrical resistivity or conductivity logs by “focused”measurements. U.S. Pat. No. 3,452,269 (assigned to the present assignee)describes an instrument adapted for taking these focused measurements.The technique described in the '269 patent uses a survey current emittedby a principal survey current emitting electrode. This survey current isconfined to a path substantially perpendicular to the borehole axis byfocusing currents emitted from nearby focusing electrodes. U.S. Pat. No.3,305,771 describes a focusing technique using an instrument equippedwith toroidal coils. U.S. Pat. Nos. 3,772,589, 4,087,740, 4,286,217 (allassigned to the present assignee) describe other electrode-typeinstruments used for subsurface measurements.

[0009] U.S. Pat. No. 5,426,368 (assigned to the present assignee)describes a logging technique using an array of current electrodesdisposed on a support. The '368 patent uses the electrode configurationto investigate the geometrical characteristics of the borehole and theresistivity properties of the formation. U.S. Pat. Nos. 5,235,285 and5,339,037 (both assigned to the present assignee) describe metallicinstruments adapted with a toroidal coil and electrode system forobtaining resistivity measurements while drilling. The measurementtechniques described in the '285 and '037 patents entail inducing acurrent that travels in a path including the conductive support body andthe formation.

[0010] U.S. Pat. Nos. 3,388,325 and 3,329,889 (both assigned to thepresent assignee) describe instruments equipped with an electrode andcoil configuration for obtaining subsurface measurements. U.S. Pat. No.3,760,260 (assigned to the present assignee) also describes a downholeinstrument equipped with electrodes and coils. The '260 patent uses theelectrode configuration to ensure radial current flow into the formationsurrounding the borehole. U.S. Pat. No. 4,511,843 (assigned to thepresent assignee) describes a logging technique whereby currents areemitted from electrodes to zero a potential difference between otherelectrodes on the instrument. U.S. Pat. No. 4,538,109 (assigned to thepresent assignee) describes a logging technique aimed at correcting orcanceling the effects of spurious EM components on downhole measurementsignals.

[0011] A coil carrying a current can be represented as a magnetic dipolehaving a magnetic moment proportional to the current and the area. Thedirection and strength of the magnetic moment can be represented by avector perpendicular to the plane of the coil. In conventional inductionand propagation logging instruments, the transmitter and receiverantennas are mounted with their axes along the longitudinal axis of theinstrument. Thus, these instruments are implemented with antennas havinglongitudinal magnetic dipoles (LMD). When such an antenna is placed in aborehole and energized to transmit EM energy, currents flow around theantenna in the borehole and in the surrounding formation. There is nonet current flow up or down the borehole.

[0012] An emerging technique in the field of well logging is the use ofinstruments incorporating antennas having tilted or transverse coils,i.e., where the coil's axis is not parallel to the support axis. Theseinstruments are thus implemented with antennas having a transverse ortilted magnetic dipole (TMD). The aim of these TMD configurations is toprovide EM measurements with directed sensitivity and sensitivity to theanisotropic resistivity properties of the formation. Logging instrumentsequipped with TMDs are described in U.S. Pat. Nos. 4,319,191, 5,508,616,5,757,191, 5,781,436, 6,044,325, 6,147,496, WO 00/50926, and in V. F.Mechetin et al., TEMP—A New Dual Electromagnetic and LaterologApparatus-Technological Complex, Thirteenth European FormationEvaluation Symposium Transactions, Budapest Chapter, paper K, 1990.

[0013] A particularly troublesome property of the TMD is the extremelylarge borehole effect that occurs in high contrast situations, i.e.,when the mud in the borehole is much more conductive than the formation.When a TMD is placed in the center of a borehole, there is no netcurrent along the borehole axis. When it is eccentered in a directionparallel to the direction of the magnetic moment, the symmetry of thesituation insures that there is still no net current along the boreholeaxis. However, when a TMD is eccentered in a direction perpendicular tothe direction of the magnetic moment, axial currents are induced in theborehole. In high contrast situations these currents can flow for a verylong distance along the borehole. When these currents pass by TMDreceivers, they can cause undesired signals that are many times largerthan would appear in a homogeneous formation without a borehole.

[0014] U.S. Pat. No. 5,041,975 (assigned to the present assignee)describes a technique for processing signal data from downholemeasurements in an effort to correct for borehole effects. U.S. Pat. No.5,058,077 describes a technique for processing downhole sensor data inan effort to compensate for the effect of eccentric rotation on thesensor while drilling. However, neither of these patents relates to theproperties or effects of TMDs in subsurface measurements.

[0015] Thus there remains a need for improved methods and apparatus forreducing or correcting for these currents when using well logginginstruments implemented with TMDS.

2. SUMMARY OF THE INVENTION

[0016] An embodiment of the invention provides an apparatus for use in aborehole traversing a formation. The apparatus comprises an elongatedsupport having a longitudinal axis; at least one antenna disposed on thesupport such that the magnetic dipole moment of the antenna is tilted orperpendicular with respect to the longitudinal axis of the support. Eachantenna is adapted to transmit and/or receive electromagnetic energy.First and second electrodes are disposed on the support, with the secondelectrode being disposed such that at least one antenna is locatedbetween the first and second electrode. The first electrode is coupledto the second electrode to provide a path for a current between theelectrodes.

[0017] Another embodiment of the invention provides an apparatus for usein a borehole traversing a formation. The apparatus comprises anelongated non-conductive support having a longitudinal axis and at leastone conductive segment disposed thereon. At least one antenna isdisposed on the support such that the magnetic dipole moment of theantenna is tilted or perpendicular with respect to the longitudinal axisof the support. The at least one antenna is disposed along a conductivesegment on the support; and each antenna is adapted to transmit and/orreceive electromagnetic energy.

[0018] Another embodiment of the invention provides an apparatus for usein a borehole traversing a formation. The apparatus comprises anelongated support having a longitudinal axis and at least one antennadisposed on the support such that the magnetic dipole moment of theantenna is tilted or perpendicular with respect to the longitudinal axisof the support. Each at least one antenna is adapted to transmit and/orreceive electromagnetic energy. A first pair of electrodes is disposedon the support and adapted for joint electromagnetic interaction. Thefirst pair of electrodes is disposed such that the at least one antennais located between the electrodes. A second pair of electrodes isdisposed on the support and adapted for joint electromagneticinteraction. The second pair of electrodes is disposed such that thefirst electrode pair is located between the second electrode pair.

[0019] Another embodiment of the invention provides a method foraltering the flow of an axial electric current along a subsurfaceborehole in the vicinity of an antenna disposed within the borehole, theantenna being disposed such that the magnetic dipole moment of theantenna is tilted or perpendicular with respect to the borehole axis andbeing adapted to transmit and/or receive electromagnetic energy. Themethod comprises providing first conductive means within the borehole;providing second conductive means within the borehole, the secondconductive means positioned such that the antenna is located between thefirst and second conductive means; and coupling the first and secondconductive means to provide a path through the antenna for the axialcurrent to flow between the conductive means.

[0020] Another embodiment of the invention provides a method foraltering the flow of an axial electric current along a subsurfaceborehole in the vicinity of an antenna disposed within the borehole, theantenna being disposed on a non-conductive support having a longitudinalaxis and adapted for disposal within the borehole, the antenna beingadapted to transmit and/or receive electromagnetic energy. The methodcomprises mounting a conductive segment on the support such that thesegment is exposed to the borehole when the support is disposed withinthe borehole; disposing the antenna along the conductive segment toprovide a path through the antenna for the axial current flow when thesupport is disposed within the borehole; and disposing the antenna alongthe conductive segment such that the magnetic dipole moment of theantenna is tilted or perpendicular with respect to the longitudinal axisof the support.

[0021] Another embodiment of the invention provides a method foraltering the flow of an axial electric current along a subsurfaceborehole in the vicinity of an antenna disposed within the borehole, theantenna being disposed such that the magnetic dipole moment of theantenna is tilted or perpendicular with respect to the borehole axis andbeing adapted to transmit and/or receive electromagnetic energy. Themethod comprises disposing a first pair of electrodes within theborehole such that the antenna is located between the electrodes, thefirst electrode pair being adapted for joint electromagneticinteraction; disposing a second pair of electrodes within the boreholesuch that the first electrode pair is located between the secondelectrode pair, the second electrode pair being adapted for jointelectromagnetic interaction; measuring an electromagnetic propertyassociated with the axial electric current at the first or secondelectrode pair; and emitting a current within the borehole in responseto the measured electromagnetic property, the current being emittedbetween: i) the first electrode pair if the second electrode pair wasused in the measurement of the electromagnetic property; or ii) thesecond electrode pair if the first electrode pair was used in themeasurement of the electromagnetic property.

[0022] Another embodiment of the invention provides a method forcorrecting for the effect of an axial electric current flow along asubsurface borehole in the vicinity of a receiver antenna disposedwithin the borehole, the receiver antenna being disposed such that themagnetic dipole moment of the antenna is tilted or perpendicular withrespect to the borehole axis, the axial current being associated withelectromagnetic energy transmitted from a transmitter antenna disposedwithin the borehole. The method comprises disposing a first pair ofelectrodes within the borehole such that the antenna is located betweenthe electrodes, the first electrode pair being adapted for jointelectromagnetic interaction; disposing a second pair of electrodeswithin the borehole such that the first electrode pair is locatedbetween the second electrode pair, the second electrode pair beingadapted for joint electromagnetic interaction; measuring a voltagesignal associated with the transmitted electromagnetic energy at thereceiver antenna; measuring a voltage difference between the first orsecond electrode pair, the voltage difference being associated with thetransmitted electromagnetic energy; shutting off transmission of theelectromagnetic energy from the transmitter antenna; emitting a currentwithin the borehole, the current being emitted between: i) the firstelectrode pair if the second electrode pair was used in the voltagedifference measurement; or ii) the second electrode pair if the firstelectrode pair was used in the voltage difference measurement; measuringa voltage signal associated with the emitted current at the receiverantenna; measuring a voltage difference between the electrode pair usedin the previous voltage difference measurement, the voltage differencebeing associated with the emitted current; and calculating a voltagevalue based on the measured voltage signals and voltage differences.

[0023] Another embodiment of the invention provides an apparatus forinduction logging within a borehole traversing a formation. Theapparatus comprises an elongated conductive metal body having alongitudinal axis and at least one antenna disposed on the body suchthat the magnetic dipole moment of the antenna is tilted orperpendicular with respect to the longitudinal axis of the body. Eachantenna is adapted to transmit and/or receive electromagnetic energy forelectromagnetic exploration of the formation.

[0024] Another embodiment of the invention provides a method foraltering the flow of an axial electric current along a subsurfaceborehole in the vicinity of an antenna disposed within the borehole, theantenna being disposed on a conductive metal body having a longitudinalaxis, the antenna being adapted to transmit and/or receiveelectromagnetic energy at a frequency range of 1 kHz to 5 MHz. Themethod comprises disposing the antenna along the metal body such thatthe magnetic dipole moment of the antenna is tilted or perpendicularwith respect to the longitudinal axis of the body; and disposing themetal body within the borehole such that the body is exposed to theborehole to provide a path for the axial current flow.

3. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other aspects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

[0026]FIG. 1 shows a pictorial looking downhole of parallel andperpendicular eccentering of a tilted or transverse magnetic dipolewithin a borehole.

[0027]FIG. 2 is a schematic diagram of an instrument with an arrayedelectrode configuration according to the invention.

[0028]FIG. 3 is a schematic diagram of an instrument with an annularelectrode configuration according to the invention.

[0029]FIG. 4 is a schematic diagram of an instrument with a conductivesegment disposed on a non-conductive support according to the invention.

[0030]FIG. 5 is a schematic diagram illustrating the current pathsencountered with a conductive all-metal instrument having aperpendicularly eccentered tilted or transverse magnetic dipole inaccord with the invention.

[0031]FIG. 6 is a schematic diagram of an instrument with multipleelectrode pairs configured about an antenna according to the invention.

[0032]FIG. 7 shows a flow chart of an embodiment of a method accordingto the invention.

[0033]FIG. 8 is a schematic diagram of the induced axial current flowencountered in the borehole with a non-conductive instrument having aperpendicularly eccentered tilted or transverse magnetic dipole.

[0034]FIG. 9 illustrates the current injected into the borehole from aninstrument equipped with electrode pairs about a source according to theinvention.

[0035]FIG. 10 is a schematic diagram of the axial current flow about aninstrument equipped with electrode pairs about a sensor according to theinvention.

4. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0036] Before proceeding with disclosure of the invention, sometheoretical consideration shall be set forth.

[0037] A TMD can be eccentered in a borehole in two possibleorientations, which we will call parallel and perpendicular eccenteringas shown in FIG. 1. Parallel eccentering forces currents symmetricallyup and down the borehole and therefore no net current is generated. Thisborehole effect is no worse than in a typical downhole instrumentequipped with non-tilted (axial) antennas. Perpendicular eccenteringgives rise to a large axial borehole current in the case of an insulatedinstrument body, which strongly couples to a transverse receiver anaxial distance away (not shown). These two displacements are theextremes of the possible ones. In the general case, the eccentering willbe in a direction that is at some angle to the dipole moment of thesensors. In this case, the borehole effect lies between the two extremecases.

[0038] It is important to understand the basic difference between theborehole effect of a conventional LMD and the borehole effect of a TMD.If either type of source is placed in a homogeneous medium, currentswill flow in paths surrounding the transmitter. When a borehole isadded, these current paths are distorted. These currents induce avoltage in a receiver coil displaced from the transmitter. This voltageis an indication of the resistivity of the formation. If instead of ahomogeneous medium, we include a borehole, then the current paths arealtered and hence the received voltage is different from what would bemeasured in the absence of a borehole. This difference is called the“borehole effect.” The difference in borehole effect between an LMD anda TMD is due to the difference between the distortion of the currents inthe presence of a borehole. With an LMD centered or eccentered in aborehole, these currents flow in the borehole in a region near thetransmitter. We know that the field of a localized current distributioncan be represented as by a multipole expansion. The leading term (thedipole term) falls off as 1/r³, where r is the radial distance in anydirection away from the transmitter. Other terms fall off even faster.

[0039] For a TMD eccentered in a borehole in a direction along thedirection of the dipole moment (parallel), we have a similar situation.Currents flow up one side of the borehole and down the other in asymmetric manner. There is no net current in the borehole past thetransmitter. This localized current causes a dipole field just as withan LMD. When the TMD is eccentered in a direction perpendicular to thedirection of the dipole moment, these currents are no longer symmetricand a net current flows in the borehole past the transmitter. Thiscurrent flows up the borehole and returns through the formation. Whenthis current passes the receiver coil, a voltage is induced in the coil.This current falls off, not geometrically at least as rapidly as 1/r³,but exponentially as e^(−(z/z) ^(₀) ⁾ where z₀ is proportional to$\sqrt{\frac{R_{formation}}{R_{mud}}}$

[0040] When the borehole is much more conductive than the formation,this leads to a very slow falloff in this current.

[0041] In the case of an LMD, or a parallel eccentered TMD, the voltagein the receiver is due to the fields from the localized distortion ofthe current distribution near the transmitter. In the case of aperpendicularly eccentered TMD, it is due to the field from a currenttravelling in the borehole right past the receiver. This second effectis much larger than the first.

[0042]FIG. 2 shows an embodiment of the invention. A logging instrumentwith a non-conductive body is shown disposed within a borehole. Theinstrument is equipped with a transverse (90° axis tilt) transmitterantenna Tx and a transverse receiver antenna Rx. The instrument is alsoequipped with a pair of electrodes E₁, E₂ positioned at opposite ends ofthe transmitter antenna Tx. The electrodes E₁, E₂ may be formed as anarray of circumferentially spaced apart azimuthal metallic electrodes.FIG. 2 shows an electrode configuration composed of an array of sixteendiscrete azimuthal metallic segments 10 mounted on an insulating toroid12. Alternatively, the electrodes E₁, E₂ may also be formed as one-piecemetallic annular electrodes as shown in FIG. 3. If an annular electrodeconfiguration is used, it is preferable to leave an axial gap or openingalong the circumference of the electrode. It will be appreciated bythose skilled in the art that various types of electrode configurationsmay be used to implement the invention as known in the art, such asbutton electrodes.

[0043] The electrodes E₁, E₂ are shorted together with a conductor(e.g., a wire, cable, or metallic strap) 14 that preferably runs throughthe center of the transmitter antenna Tx. If the electrodes E₁, E₂ areconfigured as an array of circumferentially spaced apart azimuthalelectrodes, all the electrode segments of E₁ are shorted together andall the electrode segments of E₂ are shorted together and E₁ is shortedto E₂. The shorting of the electrodes E₁ and the shorting of theelectrodes E₂ is preferably done with wires that run radially to avoidthe formation of an azimuthal current loop. By shorting the electrodesE₁, E₂ above and below the transmitter Tx, this configuration insuresthat there is no net electric field along the borehole and so no netcurrent flow. The conductor(s) connecting upper and lower electrodes E₁,E₂ pass through the transmitter Tx and allow currents flowing throughthe borehole to close. This leads to a localized current distributionwithout the long-range axial currents, which would otherwise be presentin the borehole. This localized distribution of currents has, at most, adipole moment which falls off at least as fast as 1/L³, where L is thespacing between antennas. In effect, this configuration shorts theazimuthally varying induced axial current through the transmitter Tx andforms a local magnetic dipole in opposition to the TMD. The electrodesE₁, E₂ may be mounted on the instrument by any suitable means known inthe art.

[0044]FIG. 4 shows another embodiment of the invention. By mounting theTMD about a conductive segment 16 disposed along the non-conductivesupport member of the instrument, a local induced current distributionis formed. The current loop that is created is composed of the boreholeand conductive segment 16. The conductive segment 16 may be formed as ametallic tube or sleeve mounted on the non-conductive support. Theinstrument may be equipped with multiple conductive segments andantennas as desired. Modeling and experiments show that the current thatflows in the borehole and metal section of the instrument is limited inaxial extent to a few times the borehole diameter. Thus the length ofthe conductive section is variable, but preferably more than a few timesthe diameter of the largest borehole where the instrument may be run.

[0045] With the conductive segment 16 disposed in alignment with anantenna and in contact with the borehole fluid, the axial currentinduced in the borehole returns through the instrument body in thevicinity of the antenna instead of traveling for a long distance alongthe borehole. If the conductive segment 16 is about the receiverantenna, then the axial current that would otherwise travel in theborehole will instead travel in the conductive segment 16. Thus, theflow of the induced axial current along the borehole is minimized byproviding an alternate path for the current along the instrument body.An alternative embodiment extends the conductive segment 16 to thelength of the instrument (not shown), in essence consisting of afull-metal sleeve along the support.

[0046] Conventional induction logging instruments, particularly wirelineinstruments, comprise antennas in housings formed of non-conductivematerials such as fiberglass reinforced epoxy resin. FIG. 5 showsanother embodiment of the invention. A TMD antenna is disposed on alogging instrument 18 consisting of an all-metal body 20. A layer of anelectrically insulating material (e.g., Randallite, fiberglass-epoxy, orrubber) is placed between the antenna and the body 20. The instrument 18is also equipped with a signal generator mounted within the body (notshown) to pass an alternating current through the antenna. The signalgenerator operating frequency is generally between 1 kHz and 5 MHz.Alternatively, the current may be fed to the antenna through a wirelinecable as known in the art.

[0047] As shown in FIG. 5, when the instrument 18 is eccentered in theborehole, the metallic body 20 is exposed to the borehole fluid suchthat a local induced current distribution is formed along the body 20. Ashield 22 is also mounted on the body 20 to protect the TMD antenna andto permit the passage of particular desired electromagnetic energycomponents. U.S. Pat. Nos. 4,949,045 and 4,536,714 (both assigned to thepresent assignee) describe conductive metallic shield configurationsthat may be used. Those skilled in the art will appreciate that othersuitable shields may be used with the instrument 18. For example, ashield may be configured in the form of a strip (not shown), alsoreferred to as flex circuit, to provide flexibility and easy mounting.

[0048] For effective operation of the TMD antenna, the resulting currentflow should not induce a voltage in the antenna. Thus if a conductiveshield 22 is placed over the antenna so that current flows there insteadof in the borehole fluid, a zero current will be induced in the antennaif the current in the shield 22 is azimuthally symmetric. Otherwise thevoltage in the receiver antenna may be greater than it would be ifcurrent were flowing in the mud. The desired axisymmetric currentdistribution may be achieved by disposing a conductive material betweenthe shield 22 and the body 20 such that an azimuthally uniformconnection is formed. For example, a conductive metallic O-ring orgasket may be disposed at both ends of the shield 22 such that there areno breaks between the shield 22 and the body 20 (not shown). Withrespect to the embodiment of FIG. 4, the conductive segment 16 on thenon-conductive support redirects the induced current through theconductor centered through the TMD such that there will be zero voltageinduced in the TMD within the mechanical accuracy of the placement ofthe conductor.

[0049] A zero current induced in the TMD antenna is also achieved byinsulating the conductive shield 22 from the metallic body 20. This maybe attained by mounting the shield 22 on the body 20 such that one endis fully insulated (not shown). Randallite, fiberglass-epoxy, rubber, orany suitable nonconductive material or compound may be disposed betweenthe shield 22 and the body 20 to provide the desired insulation.Alternatively, the TMD may be sealed or potted onto the body 20 with arubber over-molding or any suitable non-conductive compound that permitsthe passage of EM energy. Yet another embodiment comprises a shield 22made of an insulating material to permit the passage of EM radiation.Useable materials include the class of polyetherketones described inU.S. Pat. Nos. 4,320,224 and 6,084,052 (assigned to the presentassignee), or other suitable resins. Victrex USA, Inc. of West Chester,Pa. manufactures one type called PEEK. Cytec Fiberite, Greene Tweed, andBASF market other suitable thermoplastic resin materials. Another usableinsulating material is Tetragonal Phase Zirconia ceramic (“TZP”),manufactured by Coors Ceramics of Golden, Colo.

[0050]FIG. 6 shows another embodiment of the invention. A logginginstrument with a non-conductive body is shown disposed within aborehole. The instrument is equipped with a transverse transmitterantenna Tx and a transverse receiver antenna Rx. The receiver antenna Rxis positioned between a pair of measure electrodes M, M′, which arethemselves positioned between a pair of current electrodes A, A′. Theelectrodes M, M′, A, A′ may be formed as an array of circumferentiallyspaced apart metallic electrodes or as an annular electrode as describedabove.

[0051] One embodiment of the invention involves a process using theprinciple of superposition and a digital focusing approach. Thisembodiment is shown in flow chart form in FIG. 7. This technique may beimplemented with the embodiment of FIG. 6. In this process, thetransmitter antenna Tx is activated, at 100, and the voltage signal(V_(R1)) at the receiver antenna Rx as well as the voltage difference(ΔV_(M1)) on the measure electrodes M, M′ are obtained at 105, 110. Thetransmitter antenna is then shut off, at 115, and a current is runbetween the current electrodes A, A′ at 120. The voltage at the measureelectrodes (ΔV_(M2)) and the voltage signal (V_(R2)) at the receiverantenna are again measured at 125, 130.

[0052] The excitation necessary to produce the set of voltages (ΔV_(M1))on the measure electrodes M, M′ is then calculated, at 135, and thevoltage in the receiver antenna Rx due to this excitation is computed at140. This voltage is then subtracted from the voltage actually measuredto produce the borehole-corrected signal at 145. Mathematically theequation is expressed as $\begin{matrix}{V_{Corr} = {V_{R1} - {\frac{\Delta \quad V_{M1}}{\Delta \quad V_{M2}}{V_{R2}.}}}} & (1)\end{matrix}$

[0053] This voltage should be equal to the voltage that would appear onthe receiver antenna Rx if the longitudinal current in the borehole didnot exist in a high contrast situation. Since the transmitter antenna Txoperates at some finite frequency, and all the voltages are complex(they include an amplitude and a phase shift relative to the transmittercurrent or the electrode currents), the currents injected from theelectrodes A, A′ are at the same frequency.

[0054] The instruments of the invention may be equipped withconventional electronics and circuitry to activate the sources andsensors to obtain the desired measurements as known in the art. Onceacquired, the data may be stored and/or processed downhole orcommunicated to the surface in real time via conventional telemetrysystems known in the art.

[0055]FIG. 8 illustrates the induced axial current flow encountered inthe borehole with a typical non-conductive instrument equipped with aTMD when the TMD is perpendicularly eccentered in a conductive borehole.FIG. 9 shows another embodiment of the invention. This particularembodiment entails a feedback process. The embodiment of FIG. 9 issimilar to that of FIG. 6. The measure electrodes M, M′ are adapted tosample and measure the azimuthally varying magnitude of the inducedelectric field. Current is then injected into the borehole by thecurrent electrodes A, A′ to counter or cancel the borehole currentmeasured by the measure electrodes M, M′. Thus, current is dischargedfrom the current electrodes A, A′ in such a way as to achieve thecondition that the voltage difference between M and M′ is made equal tozero. That is ΔV=V_(M)−V_(M′)=0.

[0056]FIG. 10 shows another embodiment of the invention. The embodimentshown in FIG. 10 is similar to that of FIG. 9, except that theelectrodes are disposed about a TMD receiver on a typical non-conductiveinstrument. With this configuration, the induced current flows up theborehole, enters the current electrode A′, travels up the instrument tothe second electrode A, and continues up the borehole. In the immediatevicinity of the TMD, there is no current flow in the borehole. Themeasure electrodes M and M′ provide an analog feedback to the currentelectrodes A, A′ to just cancel the borehole effect. Thus, the flow ofthe axial current along the borehole is countered with the injection ofanother current emitted within the borehole.

[0057] As known in the art, the signals measured with inductionfrequencies are affected by direct transmitter-to-receiver coupling.Therefore, the logging instruments of the invention may also includeso-called “bucking” antennas to eliminate or reduce these couplingeffects. It will also be understood by those skilled in the art that theprinciple of reciprocity provides that the electrode and/or conductivesegment configurations of the invention will work whether they areimplemented about the transmitters or receivers on the instrument. Thespacing between the electrodes and/or antennas in the direction of theborehole may also be varied for effective implementation of theinvention. In addition, the logging instruments of the invention may be“propagation” instruments in which quantities such as phase shift orattenuation could be measured between pairs of receivers.

[0058] While the methods and apparatus of this invention have beendescribed as specific embodiments, it will be apparent to those skilledin the art that other embodiments of the invention can be readilydevised which do not depart from the concept and scope of the inventionas disclosed herein. All such similar variations apparent to thoseskilled in the art are deemed to be within the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. An apparatus for use in a borehole traversing aformation, comprising: an elongated support having a longitudinal axis;at least one antenna disposed on the support such that the magneticdipole moment of the antenna is tilted or perpendicular with respect tothe longitudinal axis of the support; each at least one antenna beingadapted to transmit and/or receive electromagnetic energy; a firstelectrode disposed on the support; and a second electrode disposed onthe support, the second electrode being disposed such that at least oneantenna is located between the first and second electrode; wherein thefirst electrode is coupled to the second electrode to provide a path fora current between the electrodes.
 2. The apparatus of claim 1, whereinthe support is non-conductive in the vicinity of the at least oneantenna disposed with its magnetic dipole moment tilted or perpendicularwith respect to the longitudinal axis of the support.
 3. The apparatusof claim 1, wherein the first and/or second electrode comprises an arrayof circumferentially spaced apart azimuthal electrodes.
 4. The apparatusof claim 1, wherein the first and/or second electrode comprises anannular electrode.
 5. The apparatus of claim 1, wherein the antennalocated between the first and second electrode is disposed with itsmagnetic dipole moment tilted or perpendicular with respect to thelongitudinal axis of the support.
 6. The apparatus of claim 1, whereinthe first electrode is coupled to the second electrode by a conductor.7. The apparatus of claim 1, wherein the current path passes through theat least one antenna located between the first and second electrodes. 8.An apparatus for use in a borehole traversing a formation, comprising:an elongated non-conductive support having a longitudinal axis and atleast one conductive segment disposed thereon; at least one antennadisposed on the support such that the magnetic dipole moment of theantenna is tilted or perpendicular with respect to the longitudinal axisof the support; the at least one antenna being disposed along aconductive segment on the support; and each at least one antenna beingadapted to transmit and/or receive electromagnetic energy.
 9. Theapparatus of claim 8, wherein each at least one conductive segmentcomprises a metallic tubular coaxially disposed on the outercircumference of the support.
 10. The apparatus of claim 9, wherein eachat least one antenna disposed along a conductive segment is electricallyinsulated from the conductive segment.
 11. The apparatus of claim 10,wherein the support comprises a plurality of independent conductivesegments disposed thereon.
 12. The apparatus of claim 11, wherein thesupport comprises two independent conductive segments, each conductivesegment having an antenna disposed thereon.
 13. The apparatus of claim12, wherein the antennas disposed on the two conductive segments havetheir magnetic dipole moments tilted or perpendicular with respect tothe longitudinal axis of the support.
 14. An apparatus for use in aborehole traversing a formation, comprising: an elongated support havinga longitudinal axis; at least one antenna disposed on the support suchthat the magnetic dipole moment of the antenna is tilted orperpendicular with respect to the longitudinal axis of the support, eachat least one antenna being adapted to transmit and/or receiveelectromagnetic energy; a first pair of electrodes disposed on thesupport and adapted for joint electromagnetic interaction; the firstpair of electrodes being disposed such that the at least one antenna islocated between the electrodes; a second pair of electrodes disposed onthe support and adapted for joint electromagnetic interaction; and thesecond pair of electrodes being disposed such that the first electrodepair is located between the second electrode pair.
 15. The apparatus ofclaim 14, wherein the support is non-conductive in the vicinity of theat least one antenna disposed with its magnetic dipole moment tilted orperpendicular with respect to the longitudinal axis of the support. 16.The apparatus of claim 15, wherein the first electrode pair is adaptedto measure a voltage difference between the electrode pair.
 17. Theapparatus of claim 16, wherein an electrode of the second electrode pairis adapted to emit a current between the electrode pair.
 18. Theapparatus of claim 15, wherein an electrode of the first electrode pairis adapted to emit a current between the electrode pair.
 19. Theapparatus of claim 18, wherein the second electrode pair is adapted tomeasure a voltage difference between the electrode pair.
 20. Theapparatus of claim 14, wherein the at least one antenna located betweenthe first electrode pair is adapted to measure a voltage signal when atleast one electrode disposed on the support is excited to emit acurrent.
 21. The apparatus of claim 14, wherein the first pair ofelectrodes is adapted to measure a voltage difference between theelectrodes when an antenna disposed in the borehole is transmittingelectromagnetic energy.
 22. The apparatus of claim 21, wherein anelectrode of the second electrode pair is adapted to emit a current inresponse to the voltage difference measured at the first electrode pair.23. The apparatus of claim 14, wherein the second pair of electrodes isadapted to measure a voltage difference between the electrodes when anantenna disposed in the borehole is transmitting electromagnetic energy.24. The apparatus of claim 23, wherein an electrode of the firstelectrode pair is adapted to emit a current in response to the voltagedifference measured at the second electrode pair.
 25. An apparatus forinduction logging within a borehole traversing a formation, comprising:an elongated conductive metal body having a longitudinal axis; and atleast one antenna disposed on the body such that the magnetic dipolemoment of the antenna is tilted or perpendicular with respect to thelongitudinal axis of the body; wherein each at least one is antenna isadapted to transmit and/or receive electromagnetic energy forelectromagnetic exploration of the formation.
 26. The apparatus of claim25, further comprising means for conducting an alternating currentthrough the at least one antenna, the alternating current comprising arange of 1 kHz to 5 MHz.
 27. The apparatus of claim 26, furthercomprising a shield disposed on the body to cover the at least oneantenna, the shield permitting the passage of particular electromagneticenergy components.
 28. The apparatus of claim 27, wherein the shield isadapted such that no current flows throughout the shield.
 29. Theapparatus of claim 28, wherein the apparatus is adapted for disposalwithin the borehole on a wireline.
 30. The apparatus of claim 27,wherein the shield is metallic and adapted such that current flowthroughout the shield is azimuthally symmetric.
 31. The apparatus ofclaim 30, wherein the apparatus is adapted for disposal within theborehole on a wireline.
 32. A method for altering the flow of an axialelectric current along a subsurface borehole in the vicinity of anantenna disposed within the borehole, the antenna being disposed suchthat the magnetic dipole moment of the antenna is tilted orperpendicular with respect to the borehole axis and being adapted totransmit and/or receive electromagnetic energy, comprising: a) providingfirst conductive means within the borehole; b) providing secondconductive means within the borehole, the second conductive meanspositioned such that the antenna is located between the first and secondconductive means; and c) coupling the first and second conductive meansto provide a path through the antenna for the axial current to flowbetween the conductive means.
 33. The method of claim 32, wherein thefirst and second conductive means comprise electrodes.
 34. The method ofclaim 32, wherein the first and second conductive means are coupled by awire.
 35. A method for altering the flow of an axial electric currentalong a subsurface borehole in the vicinity of an antenna disposedwithin the borehole, the antenna being disposed on a non-conductivesupport having a longitudinal axis and adapted for disposal within theborehole, the antenna being adapted to transmit and/or receiveelectromagnetic energy, comprising: a) mounting a conductive segment onthe support such that the segment is exposed to the borehole when thesupport is disposed within the borehole; b) disposing the antenna alongthe conductive segment to provide a path through the antenna for theaxial current flow when the support is disposed within the borehole; andc) disposing the antenna along the conductive segment such that themagnetic dipole moment of the antenna is tilted or perpendicular withrespect to the longitudinal axis of the support.
 36. The method of claim35, wherein the conductive segment comprises a metallic tubularcoaxially disposed on the outer circumference of the support.
 37. Amethod for altering the flow of an axial electric current along asubsurface borehole in the vicinity of an antenna disposed within theborehole, the antenna being disposed such that the magnetic dipolemoment of the antenna is tilted or perpendicular with respect to theborehole axis and being adapted to transmit and/or receiveelectromagnetic energy, comprising: (a) disposing a first pair ofelectrodes within the borehole such that the antenna is located betweenthe electrodes, the first electrode pair being adapted for jointelectromagnetic interaction; (b) disposing a second pair of electrodeswithin the borehole such that the first electrode pair is locatedbetween the second electrode pair, the second electrode pair beingadapted for joint electromagnetic interaction; (c) measuring anelectromagnetic property associated with the axial electric current atthe first or second electrode pair; and (d) emitting a current withinthe borehole in response to the measured electromagnetic property ofstep (c), the current being emitted between: i) the first electrode pairif the second electrode pair was used in the measurement of step (c); orii) the second electrode pair if the first electrode pair was used inthe measurement of step (c).
 38. The method of claim 37, wherein step(c) comprises measuring the magnitude of an electric field induced bythe axial electric current.
 39. A method for correcting for the effectof an axial electric current flow along a subsurface borehole in thevicinity of a receiver antenna disposed within the borehole, thereceiver antenna being disposed such that the magnetic dipole moment ofthe antenna is tilted or perpendicular with respect to the boreholeaxis, the axial current being associated with electromagnetic energytransmitted from a transmitter antenna disposed within the borehole,comprising: (a) disposing a first pair of electrodes within the boreholesuch that the antenna is located between the electrodes, the firstelectrode pair being adapted for joint electromagnetic interaction; (b)disposing a second pair of electrodes within the borehole such that thefirst electrode pair is located between the second electrode pair, thesecond electrode pair being adapted for joint electromagneticinteraction. (c) measuring a voltage signal associated with thetransmitted electromagnetic energy at the receiver antenna; (d)measuring a voltage difference between the first or second electrodepair, the voltage difference being associated with the transmittedelectromagnetic energy; (e) shutting off transmission of theelectromagnetic energy from the transmitter antenna; (f) emitting acurrent within the borehole, the current being emitted between: i) thefirst electrode pair if the second electrode pair was used in themeasurement of step (d); or ii) the second electrode pair if the firstelectrode pair was used in the measurement of step (d); (g) measuring avoltage signal associated with the emitted current at the receiverantenna; (h) measuring a voltage difference between the electrode pairused in the measurement of step (d), the voltage difference beingassociated with the emitted current; and (i) calculating a voltage valuebased on the measured voltage signals and voltage differences.
 40. Amethod for altering the flow of an axial electric current along asubsurface borehole in the vicinity of an antenna disposed within theborehole, the antenna being disposed on a conductive metal body having alongitudinal axis, the antenna being adapted to transmit and/or receiveelectromagnetic energy at a frequency range of 1 kHz to 5 MHz,comprising: a) disposing the antenna along the metal body such that themagnetic dipole moment of the antenna is tilted or perpendicular withrespect to the longitudinal axis of the body; and b) disposing the metalbody within the borehole such that the body is exposed to the boreholeto provide a path for the axial current flow.
 41. The method of claim40, further comprising shielding the at least one antenna to permit thepassage of particular electromagnetic energy components.
 42. The methodof claim 41, the shielding step comprising covering the at least oneantenna with a shield adapted such that no current flows throughout theshield.
 43. The method of claim 42, wherein the method is performedafter drilling of the borehole.
 44. The method of claim 41, theshielding step comprising covering the at least one antenna with ametallic shield adapted such that current flow throughout the shield isazimuthally symmetric.
 45. The method of claim 44, wherein the method isperformed after drilling of the borehole.